This is a little more complex than you might think. First and foremost, you'll need a source to obtain current conditions for a given location. If you're in the US, the NWS has web services you can call to obtain current conditions of a given station; however, knowing which station is closest to the zip they entered is tricky. You'll need to know the latitude and longitude of the station and the zipcode to determine the closest station.
Getting it to display "The humidly will be high today" is the easy part. That's just a simple conditional statement once you have the humidity value.